The present invention relates to circuits for achieving close to unity power factor for electrical loads and more particularly to dynamic power factor control and correction for switching power supplies.
"Power factor" is defined as the ratio of the input power in watts (actual power) over the power (V.sub.rms .times.I.sub.rms) measured with an rms voltmeter and ammeter (apparent power). The traditional concept of phase angle between the voltage and current waveforms used to define power factor is not applicable to switching power supplies for off-line converters because the input current is drawn in short pulses of high peak value such that the current waveform is not sinusoidal. Therefore, traditional methods of improving power factor with passive components such as inductors and capacitors do not work with switching power supplies or off-line converters because the current is drawn in the form of narrow pulses having high peak values. To achieve a high power factor in switching power supplies, the line current is chopped at a relatively high frequency and fed to a fly-back type of circuit. The amount of current that is allowed to feed the fly-back circuit is electronically controlled such that the average current forms a sinusoidal waveform of a proportional magnitude and identical phase as the line voltage.
Various forms of power factor correction for off-line switching power supplies have been proposed in the prior art. These prior art devices switch the current through the fly-back transformer or inductor while the inductor is charged. This dynamic switching of inductor current results in switching losses and heating of the various components. Some of these devices use a feedback sense signal from the incoming line to attempt to match the switching of the inductor current to the sinusoidal line voltage when then limits the operation of the circuit to AC input only. Some of these circuits also are limited as to their voltage operation and frequency of operation range, forcing design changes to adapt the unit to various international standards for units shipped overseas. The efficiency of operation of these prior art switching power supplies is limited by the fact that a fixed chopping frequency is used at all points along the sinusoidal line voltage curve.
The present invention is designed to obtain a high power factor in the range of greater than 99% by controlling the current through the fly-back circuit and shaping the current demand of the circuit to match the sinusoidal waveform of the input line voltage. The circuit is designed to operate from a wide range of line voltages and frequencies and includes integral current limiting and voltage sensing to produce a highly stable output voltage at high current demands. Thus, the present invention overcomes many of the drawbacks of the prior art in an efficient and cost-effective manner.